U.S. patent application number 11/017489 was filed with the patent office on 2006-06-22 for method and system for spatial-spectral excitation by parallel rf transmission.
This patent application is currently assigned to General Electric Company. Invention is credited to Christopher Judson Hardy, Yudong Zhu.
Application Number | 20060132133 11/017489 |
Document ID | / |
Family ID | 36594842 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060132133 |
Kind Code |
A1 |
Zhu; Yudong ; et
al. |
June 22, 2006 |
Method and system for spatial-spectral excitation by parallel RF
transmission
Abstract
A magnetic resonance imaging (MRI) system and method is
provided. The MRI system comprises a plurality of transmit coils
arranged spatially distinct from each other and configured for
inducing a nuclear magnetic resonance (NMR) excitation. The NMR
excitation is selective both in spatial dimensions and in a
chemical shift spectrum. The plurality of transmit coils are driven
by a plurality of radio frequency (RF) pulses, and a gradient
module is driven by a plurality of gradient pulses.
Inventors: |
Zhu; Yudong; (Clifton Park,
NY) ; Hardy; Christopher Judson; (Schenectady,
NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
General Electric Company
|
Family ID: |
36594842 |
Appl. No.: |
11/017489 |
Filed: |
December 20, 2004 |
Current U.S.
Class: |
324/318 ;
324/307 |
Current CPC
Class: |
G01R 33/3415 20130101;
G01R 33/5611 20130101; G01R 33/446 20130101 |
Class at
Publication: |
324/318 ;
324/307 |
International
Class: |
G01V 3/00 20060101
G01V003/00 |
Claims
1. A magnetic resonance imaging (MRI) system comprising: a
plurality of radio frequency (RF) transmit coils and a gradient
module, the plurality of RF transmit coils being arranged spatially
distinct from each other and configured for inducing a nuclear
magnetic resonance (NMR) excitation, the NMR excitation being
selective in spatial dimensions and in a chemical-shift spectrum,
wherein the plurality of transmit coils are driven by a plurality
of radio frequency (RF) pulses concurrently with gradient pulses
applied to the gradient module.
2. The MRI system of claim 1, wherein the NMR excitation is further
configured to accelerate k-space sampling along one
spatial-frequency axis.
3. The MRI system of claim 1, wherein NMR excitation is configured
to accelerate k-space sampling along two spatial-frequency
axes.
4. The MRI system of claim 1 wherein NMR excitation is configured
to accelerate k-space sampling along three spatial-frequency
axes.
5. The MRI system of claim 1, wherein the NMR excitation is
simultaneously selective in one spatial dimension and one frequency
dimension.
6. The MRI system of claim 1, wherein the NMR excitation is
simultaneously selective in two spatial dimensions and one
frequency dimension.
7. The MRI system of claim 1 wherein the NMR excitation is
simultaneously selective in three spatial dimensions and one
frequency dimension.
8. The MRI system of claim 1, wherein the RF pulses and gradient
pulses are further configured to effect a reduction in k-space
sampling density during excitation.
9. The MRI system of claim 8, wherein the reduced sampling density
results in a reduced pulse length of the NMR excitation.
10. The MRI system of claim 9, wherein a k-space sampling
trajectory comprises a repeated spiral-out and spiral-in
pattern.
11. A method for magnetic resonance imaging (MRI), the method
comprising: controlling respective currents in multiple transmit
coils to induce a nuclear magnetic resonance (NMR) excitation over
a selected spatial region of an object and a selected band of
resonance frequencies.
12. The method of claim 11, further comprising accelerating k-space
sampling along one spatial-frequency axis.
13. The method of claim 11, further comprising accelerating k-space
sampling along two spatial-frequency axes.
14. The method of claim 11 further comprising accelerating k-space
sampling along three spatial-frequency axes.
15. The method of claim 11, wherein the NMR excitation is
simultaneously selective in one spatial dimension and one frequency
dimension.
16. The method of claim 11, wherein the NMR excitation is
simultaneously selective in two spatial dimensions and one
frequency dimension.
17. The method of claim 11 wherein the NMR excitation is
simultaneously selective in three spatial dimensions and one
frequency dimension.
18. The method of claim 11, further comprising reducing k-space
sampling density during excitation.
19. The method of claim 18, wherein a k-space trajectory assumes a
repeated spiral-out and spiral-in pattern.
Description
BACKGROUND
[0001] This invention relates generally to magnetic resonance
imaging (MRI), and more particularly, to transmit coil arrays used
in MRI.
[0002] Generally, MRI is a well-known imaging technique. A
conventional MRI device establishes a homogenous magnetic field,
for example, along an axis of a person's body that is to undergo
MRI. The homogeneous magnetic field conditions the interior of the
person's body for imaging by aligning the nuclear spins of nuclei
(in atoms and molecules forming the body tissue) along the axis of
the magnetic field. If the orientation of the nuclear spin is
perturbed out of alignment with the magnetic field, the nuclei
attempt to realign their nuclear spins with an axis of the magnetic
field. Perturbation of the orientation of nuclear spins may be
caused by application of radio frequency (RF) pulses. During the
realignment process, the nuclei precess about the axis of the
magnetic field and emit electromagnetic signals that may be
detected by one or more coils placed on or about the person.
[0003] The frequency of the magnetic resonance (MR) signal emitted
by a given precessing nucleus depends on the strength of the
magnetic field at the nucleus' location. As is well known in the
art, it is possible to distinguish radiation originating from
different locations within the person's body by applying a field
gradient to the magnetic field across the person's body. For the
sake of convenience, direction of this field gradient may be
referred to as the left-to-right direction. Radiation of a
particular frequency may be assumed to originate at a given
position within the field gradient, and hence at a given
left-to-right position within the person's body. The application of
such a field gradient is also referred to as frequency
encoding.
[0004] However, the application of a field gradient does not allow
for two-dimensional resolution, since all nuclei at a given
left-to-right position experience the same field strength, and
hence emit radiation of the same frequency. Accordingly, the
application of a frequency-encoding gradient, by itself, does not
make it possible to discern radiation originating from the top
versus radiation originating from the bottom of the person at a
given left-to-right position. Resolution has been found to be
possible in this second direction by application of gradients of
varied strength in a perpendicular direction to thereby perturb the
nuclei in varied amounts. The application of such additional
gradients is also referred to as phase encoding.
[0005] Frequency-encoded data sensed by the coils during a phase
encoding step is stored as a line of data in a data matrix known as
the k-space matrix. Multiple phase encoding steps are performed in
order to fill the multiple lines of the k-space matrix. An image
may be generated by using various imaging applications where a
Fourier transformation of the k-space matrix is performed to
convert frequency information to spatial information representing
the distribution of nuclear spins or density of nuclei of the image
material.
[0006] In many imaging applications, examination of the object with
spatial-spectral selectivity (that is, imaging a particular
spectral component in a particular region-of-interest) is desired
in order to meet both clinical needs (such as water/fat imaging for
examining atherosclerotic plaques and reduced-FOV imaging for
accelerating scans) as well as quality requirements (for example,
reduction of image artifacts due to frequency shifts in SSFP and
fast GRE sequences).
[0007] To induce spatial-spectral selectivity, nuclear magnetic
resonance (NMR) excitation that induces transverse magnetization to
all spins of a prescribed Larmor frequency range in a prescribed
region of interest can be used. However, with existing methods
where a volume transmit coil that effects a relatively uniform RF
field (e.g., a birdcage coil with quadrature driving) is used for
RF transmission, an NMR excitation that achieves spatial-spectral
selectivity often involves intensified pulsing activity, which
require powerful gradients to keep pulse duration in check. On
clinical scanners limitations in gradient strength or switching
rate usually render impractical the use of spatial-spectral pulses
that are selective along multiple spatial dimensions.
[0008] What is needed is a method and system to enable acceleration
of multi-dimensional spatial spectral selective pulses during
excitation.
BRIEF DESCRIPTION
[0009] Briefly, in one embodiment of the invention, a magnetic
resonance imaging (MRI) system is provided. The MRI system
comprises a plurality of radio frequency (RF) transmit coils and a
gradient module. The plurality of RF transmit coils are arranged
spatially distinct from each other and configured for inducing a
nuclear magnetic resonance (NMR) excitation, the NMR excitation
being selective both in spatial dimensions and in the chemical
shift spectrum. The plurality of transmit coils are driven by a
plurality of radio frequency (RF) pulses concurrently with gradient
pulses applied to the gradient module.
[0010] In another embodiment, a method for magnetic resonance
imaging (MRI) is provided. The method comprises controlling
respective currents in multiple transmit coils to induce a nuclear
magnetic resonance (NMR) excitation over a selected spatial region
of an object and a selected band of resonance frequencies.
DRAWINGS
[0011] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0012] FIG. 1 illustrates a simplified block diagram of a Magnetic
Resonance Imaging system to which embodiments of the present
invention are useful;
[0013] FIG. 2 is a graph illustrating exemplary gradient pulses
that are used in an embodiment of an MRI system;
[0014] FIG. 3 is diagrammatic view illustrating one manner in which
transmit coils are arranged around an object;
[0015] FIG. 4 is a graph illustrating a k-space trajectory
according to one aspect of the invention.
DETAILED DESCRIPTION
[0016] FIG. 1 illustrates a simplified block diagram of a system
for producing images in accordance with embodiments of the present
invention. In an embodiment, the system is an MR imaging system
that incorporates embodiments of the present invention. The MR
system is adapted to perform the method of the present invention,
although other systems could be used as well.
[0017] The operation of the MR system is controlled from an
operator console 100 which includes a keyboard and control panel
102 and a display 104. The console 100 communicates through a link
116 with a separate computer system 107 that enables an operator to
control the production and display of images on the screen 104. The
computer system 107 includes a number of modules which communicate
with each other through a backplane. These include an image
processor module 106, a CPU module 108, and a memory module 113,
known in the art as a frame buffer for storing image data arrays.
The computer system 107 is linked to a disk storage 111 and a tape
drive 112 for storage of image data and programs, and it
communicates with a separate system control 122 through a high
speed serial link 115.
[0018] The system control 122 includes a set of modules connected
together by a backplane. These include a CPU module 119 and a pulse
generator module 121 connected to the operator console 100 through
a serial link 125. It is through this link 125 that the system
control 122 receives commands from the operator that indicate the
scan sequence that is to be performed.
[0019] The pulse generator module 121 operates the system
components to carry out the desired scan sequence. It produces data
that indicate the timing, strength, and shape of the radio
frequency (RF) pulses which are to be produced, and the timing of
and length of the data acquisition window. The pulse generator
module 121 connects to a set of gradient amplifiers 127, to
indicate the timing and shape of the gradient pulses to be produced
during the scan. The pulse generator module 121 also receives
subject data from a physiological acquisition controller 129 that
receives signals from a number of different sensors connected to
the subject 200, such as ECG signals from electrodes or respiratory
signals from a bellows. And finally, the pulse generator module 121
connects to a scan room interface circuit 133 which receives
signals from various sensors associated with the condition of the
subject 200 and the magnet system. It is also through the scan room
interface circuit 133 that a positioning device 134 receives
commands to move the subject 200 to the desired position for the
scan.
[0020] The gradient waveforms produced by the pulse generator
module 121 are applied to a gradient amplifier system 127 comprised
of Gx, Gy and Gz amplifiers. Each gradient coil is excited by a
corresponding gradient pulse to produce the magnetic field gradient
used for spatial encoding during RF transmission and signal
acquisition. The gradient pulses driving the Gx and Gy coils during
RF transmission are illustrated in FIG. 2.
[0021] Continuing with FIG. 1, the gradient module 139 forms part
of a magnet assembly 141, which includes a polarizing magnet 140,
and a RF coil system 152. In one embodiment, the RF coil 152
comprises a plurality of transmit coils arranged spatially distinct
from each other. The manner in which the transmit coils 202-207 are
arranged around object 200 is illustrated in FIG. 3.
[0022] The transmit coils are configured for inducing a nuclear
magnetic resonance (NMR) excitation on a region of interest on
object 200. The transmit coils are driven by a plurality of radio
frequency (RF) pulses generated by pulse generator 121. In
embodiments of the present invention, the RF pulses and the
gradient pulses are configured to realize a spatial-spectral
selectivity.
[0023] For example, where an NMR excitation is applied to achieve
selectivity in (x, y, f) (i.e., selective along x axis, y axis, and
chemical shift frequency axes) with the conventional method of
volume coil transmit. It is understood, based on the k-space
interpretation of selective excitation, that the constant-rate
traversing with time in the k.sub.f direction calls for oscillation
of spatial-selection gradients, as an adequate sampling of
(k.sub.x, k.sub.y, k.sub.f) space requires repeated traversing of
appropriate (k.sub.x, k.sub.y) neighborhoods during the course of
the RF pulse. The number of repetitions or oscillations, which can
be substantial in cases involving adequate spectral selectivity
profiles, implies multi-fold increase in pulse length compared to
that of a counter-part non-spectral pulse. This, compounded with
the time-consuming (k.sub.x,k.sub.y) traversing with the
conventional method, typically leads to a prohibitively long 3D
pulse that has very limited clinical utility.
[0024] In one embodiment of the invention, the oscillation of
spatial-selection gradients is maintained whereas the
(k.sub.x,k.sub.y) traversing is substantially accelerated through
sampling density reduction, which results in reduction of the
overall pulse length. An appropriate design of the parallel RF
pulses works with the spatial domain weighting (due to the B1
patterns associated with the multiple transmit elements) and the
aliasing pattern (as determined by the pulsing gradient) to cause
the reduction or annihilation of aliasing lobes' net amplitudes
with the present parallel excitation approach.
[0025] FIG. 4 illustrates one exemplary k-space sampling scheme for
the invention. The (k.sub.x, k.sub.y, k.sub.f) space is traversed
at a constant rate along k.sub.f while with repeated spiral in and
out across k.sub.x-k.sub.y, Same to a design with the conventional
single-coil transmit approach, the extent of the spiral matches
resolution requirement on the spatial selectivity profile. On the
other hand, the sampling density across k.sub.x-k.sub.y required of
the spiral is substantially reduced with the support of the
parallel transmit coils. The reduction offsets the increase in
pulse-length due to the repetition along k.sub.f and offers an
effective control over the total length of the 3D pulse. Other
examples of the spiral scheme include repeated radial scanning
across k.sub.x-k.sub.y, etc.
[0026] In a specific embodiment of the present invention, the RF
pulses and gradient pulses induce NMR excitation so as to
accelerate k-space sampling along one spatial-frequency axis. In
another embodiment, the RF pulses and gradient pulses induce NMR
excitation so as to accelerate k-space sampling along two
spatial-frequency axes. In an alternate embodiment the RF pulses
and gradient pulses induce NMR excitation so as to accelerate
k-space sampling along three spatial-frequency axes.
[0027] The RF pulses are further configured to allow k-space
sampling density reduction along spatial-frequency axes. The
reduction of sampling density results in accelerated k-space
traversing of the RF pulses. The accelerated k-space traversing of
RF pulses reduces an overall pulse length of the RF pulses. A
k-space trajectory if viewed in a two spatial-frequency dimensions,
assumes a repeated spiral-out and spiral-in pattern as shown in
FIG. 4.
[0028] Continuing with FIG. 1, volume 142 is shown as the area
within magnet assembly 141 for receiving subject 200 and includes a
patient bore. As used herein, the usable volume of a MRI scanner is
defined generally as the volume within volume 142 that is a
contiguous area inside the patient bore where homogeneity of main,
gradient and RF fields are within known, acceptable ranges for
imaging.
[0029] A transceiver module 150 in the system control 122 produces
pulses that are amplified by a RF amplifier system 151 and coupled
to the RF coil system 152 by a transmit/receive switch system 154.
The resulting signals radiated by the excited nuclei in the subject
200 may be sensed by the same RF coil system 152 and coupled
through the transmit/receive switch system 154 to a preamplifier
system 153.
[0030] The amplified MR signals are demodulated, filtered, and
digitized in the receiver section of the transceiver 150. The
transmit/receive switch 154 is controlled by a signal from the
pulse generator module 121 to electrically connect the RF amplifier
system 151 to the coil system 152 during the transmit mode (i.e.,
during excitation) and to connect the preamplifier system 153
during the receive mode. The transmit/receive switch system 154
also enables a separate RF coil (not shown, for example, a head
coil or surface coil) to be used in either the transmit or receive
mode.
[0031] During the transmission mode, the RF pulse waveforms
produced by the pulse generator module 121 are applied to a RF
amplifier system 151 comprised of multiple amplifiers. Each
amplifier controls the current in a corresponding component coil of
the coil system 152 in accordance with the amplifier's input RF
pulse waveform. With the transmit/receive switch system 154, the RF
coil system 152 is configured to perform transmission only, or
alternatively, configured to additionally act as a receive coil
array during receive mode.
[0032] As used herein, "adapted to", "configured" and the like
refer to mechanical or structural connections between elements to
allow the elements to cooperate to provide a described effect;
these terms also refer to operation capabilities of electrical
elements such as analog or digital computers or application
specific devices (such as an application specific integrated
circuit (ASIC)) that is programmed to perform a sequel to provide
an output in response to given input signals.
[0033] The MR signals picked up by the RF coil system 152 or a
separate receive coil (not shown, for example, a body, head or
surface coil) are digitized by the transceiver module 150 and
transferred to a memory module 160 in the system control 122. When
the scan is completed and an entire array of data has been acquired
in the memory module 160, an array processor 161 operates to
Fourier transform the data into an array of image data. These image
data are conveyed through the serial link 115 to the computer
system 107 where they are stored in the disk memory 111.
[0034] In response to commands received from the operator console
100, these image data may be archived on the tape drive 112, or
they may be further processed by the image processor 106 and
conveyed to the operator console 100 and presented on the display
104. Further processing is performed by the image processor 106 to
reconstruct an image using acquired MR image data. It is to be
appreciated that a MRI scanner is designed to accomplish field
homogeneity with given scanner requirements of openness, speed and
cost.
[0035] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *